In a conventional active matrix substrate of a liquid crystal display device, it is known that a drain electrode of a thin film transistor and a pixel electrode are electrically connected through a contact hole. This pixel electrode is formed on an insulation film covering the drain electrode, and the contact hole is formed on the insulation film.
FIG. 12 is a schematic plan view of a conventional active matrix substrate.
An active matrix substrate 110 comprises a plurality of thin film transistors (hereinafter, these are also referred to as “TFT”) 120, a plurality of source signal lines 114, and a plurality of gate signal lines 112. The thin film transistors 120 are arranged in matrix states. The source signal lines 114 are provided in parallel having prescribed distances between each other along the thin film transistors 120 arranged along the column direction. The gate signal lines 112 are provided in parallel having prescribed distances between each other along the thin film transistors 120 arranged along the row direction.
To the plurality of source signal lines 114, corresponding source drivers 118 are connected respectively. Each source driver 118 applies a voltage to the corresponding source signal line 114, and this voltage corresponds to video signals.
To the plurality of gate signal lines 112, corresponding gate drivers 116 are connected respectively. Each gate driver 116 applies a voltage to the corresponding gate signal line 112, and this voltage corresponds to scanning signals.
Each of the plurality of thin film transistors 120 includes a gate electrode 122, a source electrode 124, and a drain electrode 126. The gate electrode 122 is branched from the corresponding gate signal line 112, and the source electrode 124 is branched from the corresponding source signal line 114.
Each of the plurality of drain electrodes 126 is connected with a corresponding pixel electrode 130. Each pixel electrode 130 is one terminal of a corresponding pixel capacitor 128. The other terminal of each pixel capacitor 128 is a counter electrode 132 provided on a counter substrate 154 (See FIGS. 14 and 15). The counter electrode 132 is generally shared with the plurality of pixel electrodes 130.
The active matrix substrate 110 includes a display region 134 and a terminal region 136. The display region 134 contributes the display of a video, and the terminal region 136 is arranged so as to surround a periphery of the display region 134. In the display region 134, a plurality of pixel electrodes 130 and thin film transistors 120 are arranged. In the terminal region 136, a plurality of gate drivers 116 and source drivers 118 are arranged.
A conventional liquid crystal display device 170 (See FIGS. 14 and 15) comprises the active matrix substrate 110, a counter substrate 154 (See FIGS. 14 and 15), and a liquid crystal 158 (See FIG. 14). The counter substrate 154 is opposed to the active matrix substrate 110. The liquid crystal 158 is inserted between the active matrix substrate 110 and the counter substrate 154.
When each of the plurality of thin film transistors 120 is turned on or off with respect to signals applied from the gate driver 116, the voltage corresponding to video signals applied from the source driver 118 is applied to the corresponding pixel electrode 130. The orientation of the liquid crystals is controlled according to the voltages applied to the pixel electrode 130 and the counter electrode 132. Thereby, video is displayed on the liquid crystal display device.
FIG. 13 is a plan view of the conventional active matrix substrate 110.
The gate driver 116 shown in FIG. 13 includes a signal input terminal section 138 in which scanning signals are inputted from outside.
The pixel electrode 130 is connected to the drain electrode 126 through a contact hole 150.
FIG. 14 is a cross-sectional view of a conventional liquid crystal display device taken along line P-P in FIG. 13.
The conventional liquid crystal display device 170 shown in FIG. 14 includes the active matrix substrate 110, the counter substrate 154, and the liquid crystal 158. The liquid crystal 158 is inserted between the active matrix substrate 110 and the counter substrate 154.
FIG. 14 shows a cross-sectional structure of the pixel electrode 130. The pixel electrode 130 is connected to the thin film transistor 120 and the drain electrode 126 of the thin film transistor 120, in the display region 134 of the active matrix substrate 110.
The active matrix substrate 110 includes a transparent insulation substrate 139. The gate electrode 122 is formed on the transparent insulation substrate 139. A gate insulation film 140 is formed on the transparent insulation substrate 139 so as to cover the gate electrode 122.
A semiconductor layer 146 is formed on the gate electrode 122 through the gate insulation film 140. An n+ silicon (Si) layer 148 is formed on the semiconductor layer 146 so as to align with the semiconductor layer 146.
The source electrode 124 branched from the source signal line 114 is formed on a part of the gate insulation film 140 so as to cover a part of a surface of the n+ Si layer 148, and sides of the n+ Si layer 148 and the semiconductor layer 146.
The drain electrode 126 is formed on the other part of the gate insulation film 140 so as to cover the other part of the surface of the n+ Si layer 148, the other sides of the n+ Si layer 148 and the semiconductor layer 146.
The source electrode 124 and the drain electrode 126 are arranged on the surface of the n+ Si layer 148 while having the prescribed distance between each other.
The thin film transistor 120 includes the gate electrode 122, the semiconductor layer 146, the n+ Si layer 148, the source electrode 124 and the drain electrode 126.
For increasing size or improving definition of the liquid crystal display device, it is desirable to reduce the resistances of the gate signal line 112, the source signal line 114, the gate electrode 122, the source electrode 124 and the drain electrode 126. Therefore, metals having low resistance and which are easily processed are generally used as materials of these signal lines and electrodes.
The general materials used for the gate signal line 112, the source signal line 114, the gate electrode 122, the source electrode 124 and the drain electrode 126 are Al, Mo, Ti, Ta, or the like.
Mo has comparatively low specific resistance, and is easily patterned by an etching using weak acid. Therefore, Mo is preferably used as a material for the source signal line 114, the source electrode 124 and the drain electrode 126.
Although Al has the lowest specific resistance in the above-mentioned materials, Al does not preferably contact with the n+ Si layer 148, and thus a single layer of Al is not preferably used as a material for the source electrode 124 and the drain electrode 126. Therefore, when Al is used for the source signal line, a laminated structure, for example, Al/Ti, Al/Mo, or the like, is necessary.
Since Ti has a higher specific resistance than that of Al and Mo, a single layer of Ti is not preferably used as a material for the electrode and the signal line of the liquid crystal display device which is increased in size.
Since Ta has also a high specific resistance like Ti, a single layer of Ta is not preferably used.
An insulation film 152 for protecting the thin film transistor 120 is formed on the gate insulation film 140 so as to cover the other parts of surfaces of the source electrode 124, the drain electrode 126 and the n+ Si layer 148. The material of the insulation film 152 is, for example, SiNx.
The insulation film 152 includes the contact hole 150 penetrating the insulation film 152 and extending to the drain electrode 126.
The pixel electrode 130 is formed on the insulation film 152 so as to connect with the drain electrode 126 through the contact hole 150. The material of the pixel electrode 130 is transparent ITO.
In the liquid crystal display device, more particularly, the transparent TFT liquid crystal display device, the above-mentioned constitution is preferably used, that is, the constitution comprising forming the thin film transistor 120, forming the insulation film 152 so as to cover the drain electrode 126 of the thin film transistor 120, and forming the pixel electrode 130 so as to electrically connect with the drain electrode 126 through the contact hole 150 formed in the insulation film 152.
The reason why this constitution is preferably used is as follows. That is, in this constitution, since the surface formed with the pixel electrode 130 is not the same as the surface formed with the source signal line 114, it is possible to increase the surface area of the pixel electrode 130 while preventing an electrical short-circuit between the pixel electrode 130 and the source signal line 114. The pixel electrode 130 is formed on the insulation film 152, and the source signal line 114 is connected with the source electrode 124 formed under the insulation film 152.
The counter substrate 154 includes a transparent insulation substrate 156, and the counter electrode 132 provided on the transparent insulation substrate 156.
FIG. 15 is a cross-sectional view of a conventional liquid crystal display device taken along line Q-Q in FIG. 13.
In FIG. 15, a cross-sectional structure of the signal input terminal section 138 in the gate driver 116 is shown in the terminal region 136 of the active matrix substrate 110.
The gate signal line 112 is formed on the transparent insulation substrate 139. The gate insulation film 140 is formed on the transparent insulation substrate 139 so as to cover both ends of the gate signal line 112. On the gate insulation film 140, the insulation film 152 is formed.
An antioxidation film 160 is formed so as to cover sides of the gate insulation film 140 and the insulation film 152 and a part of a surface of the insulation film 152. The antioxidation film 160 is for preventing the gate signal line 112 from oxidizing which increases the resistance. At this time, the material of the antioxidation film 160 is transparent ITO, and this ITO is the same material as the pixel electrode 130.
The above-mentioned conventional active matrix substrate 110 is manufactured by the following process.
The material for forming the gate signal line 112 and the gate electrode 122 on the transparent insulation substrate 139 is deposited by a sputtering method or the like. Then, the formed layer is mask-exposed, developed, and dry-etched to form the gate signal line 112 and the gate electrode 122, which have prescribed patterns.
Then, the material for forming the gate insulation film 140 is deposited on the transparent insulation substrate 139 by a CVD method so as to cover the gate signal line 112 and the gate electrode 122.
Then, the materials for forming the semiconductor layer 146 and the material for forming the n+ Si layer 148 are deposited by a CVD method on the material for forming the gate insulation film 140. The deposited materials for forming the semiconductor layer 146 and the n+ Si layer 148 are mask-exposed, developed, and dry-etched to form the semiconductor layer 146 and the n+ Si layer 148, which have prescribed patterns.
Then, the materials for forming the source signal line 114, the source electrode 124 and the drain electrode 126 (for example, Mo) are deposited on the gate insulation film 140 so as to cover the semiconductor layer 146 and the n+ Si layer 148. Then, the formed Mo is mask-exposed, developed, and dry-etched to form Mo having a prescribed pattern.
Then, Mo between the source electrode 124 and the drain electrode 126 is wet-etched to form a channel of the thin film transistor 120. Then, the source signal line 114, the source electrode 124 and the drain electrode 126 are formed.
Then, the material for forming the insulation film 152 is deposited on the gate insulation film 140 so as to cover the other parts of surfaces of the source electrode 124, the drain electrode 126 and the n+ Si layer 148.
Then, a part of the insulation film 152 on the drain electrode 126 is removed by a dry-etching using a mixed gas of CF4 and O2, to form contact hole 150 in the insulation film 152. A part of the insulation film 152 and a part of the gate insulation film 140 are continuously removed to expose a part of the surface of the gate signal line 112. The insulation film 152 is formed at an upper part of gate signal line 112 of the terminal region 136 (FIG. 12), and the gate insulation film 140 is formed on the gate signal line 112.
Then, ITO is deposited, and the deposited ITO is mask-exposed, developed, and etched to form the pixel electrode 130 and the antioxidation film 160, which have prescribed patterns. At this time, the pixel electrode 130 is formed on the insulation film 152 so as to electrically connect with the drain electrode 126 through the contact hole 150. On the other hand, the antioxidation film 160 is formed so as to cover a part of the surface of the gate signal line 112, sides of the gate insulation film 140 and the insulation film 152, and a part of the surface of the insulation film 152.
In the signal input terminal section 138 of the terminal region 136, a part of the surface of the gate signal line 112 is exposed by removing a part of the material for forming the insulation film 152 at the upper part of the gate signal line 112, and continuously removing a part of the material for forming the gate insulation film 140 on the gate signal line 112. In the display region 134, the contact hole 150 is formed in the insulation film 152, by removing a part of the insulation film 152 on the drain electrode 126. These processes are carried out by the same dry-etching.
However, since the same dry-etching continuously removes a part of the insulation film 152 in the display region 134 and the gate insulation film 140 in the terminal region 136, a part of the drain electrode 126 arranged under the insulation film 152 and a part of the gate insulation film 140 may be also removed in addition to a part of the insulation film 152 in the display region 134.
More particularly, an etching selection ratio between the insulation film 152 and the drain electrode 126 becomes insufficient, when a mixed gas of CF4 and O2 is used for the dry-etching, SiNX is used as the material of the insulation film 152, and Mo is used as the material of the drain electrode 126. Thereby, a part of the drain electrode 126 is removed.
FIG. 16 is a cross-sectional view of the liquid crystal display device, in which a contact hole 150A penetrates not only the insulation film 152 but also the drain electrode 126 to reach in the gate insulation film 140.
As shown in FIG. 16, in a liquid crystal display device 170A, when the contact hole 150A is formed, the pixel electrode 130 contacts with the drain electrode 126 at the cross-section of the drain electrode 126. The contact hole 150A penetrates not only the insulation film 152 but also the drain electrode 126 to reach in the gate insulation film 140. The drain electrode 126 has a remarkably small cross-section area as compared with a surface area of the contact hole 150A, and thus there is a problem that an electrical connection between the pixel electrode 130 and the drain electrode 126 is insufficient.
For solving the above problems, after the drain electrode 126 is formed, ITO is deposited on the drain electrode 126 to form a protective film. The insulation film 152 is formed on this protective film, and the contact hole 150 is formed in the insulation film 152 by mask-exposing, developing and dry-etching. Then, the formed protective film protects the drain electrode 126 with respect to the dry-etching, and thus preventing the drain electrode 126 from being etched.
However, if ITO is deposited on the drain electrode 126 to form the protective film, there are new problems increasing a cost and processing.
The present invention has arisen to mitigate and/or obviate these problems, and the primary objective of the present invention is to provide a substrate, a liquid crystal display device with the same, and a manufacturing method thereof without increasing the cost and processing. In the substrate, a first electrode and a second electrode are electrically connected stably through a contact hole. The second electrode is formed on an insulation film covering the first electrode, and the contact hole is formed in the insulation film.